When a electric current is flowing in an inductor, there is energy stored in the magnetic field. Considering a pure inductor L, the instantaneous power which must be supplied to initiate the current in the inductor is

`P=vi=Li(di)/(dt)`

so the energy input to build to a final current i is given by the integral

Energy stored `= int_0^tPdt=int_0^ILidi=1/2LI^2`

`text(Energy in Magnetic Field :)`
From analysis of the energy stored in an inductor,

Energy Stored `=1/2LI^2`

`B=muN/l i`

the energy density (energy/volume) is `=(1/2LI^2)/(Al)`

`=(1/2\(muN^2A)/l\(B^2l^2)/(mu^2N^2))/(Al)`

so the energy density stored in the magnetic field is

`eta_B=text(energy)/(volume)=1/2\B^2/mu`